1. Field of the Invention
The present invention relates to piston type compressors that convert rotation of a drive shaft to linear reciprocation of a piston with a swash plate, and more particularly, to pistons used in such compressors.
2. Description of the Related Art
Compressors are employed in air-conditioning systems for vehicles. Piston type compressors are used in such systems. As shown in FIG. 7, a typical piston type compressor is provided with a driving body, such as a swash plate 100. The swash plate 100 is fitted on a drive shaft 104 in a crank chamber 103 defined between a cylinder block 101 and a front housing 102. The swash plate 100 is inclinable with respect to the drive shaft 104 while rotating integrally with the drive shaft 104. A piston 105 is accommodated in a cylinder bore 101a, which is defined in the cylinder block 101. The piston 105 has a skirt 105a that projects toward the crank chamber 103. A slot 105b is provided in the skirt 105a. A pair of shoes 106 are slidably fitted in the opposing walls of the slot 105b. The peripheral portion of the swash plate 100 is slidably held between the shoes 106.
The rotation of the drive shaft 104 is converted to linear reciprocation of the piston 105 in the cylinder bore 110a by means of the swash plate 100 and the shoes 106. The reciprocation of the piston 105 draws refrigerant gas into the cylinder bore 101a from a suction chamber (not shown), compresses the gas in the cylinder bore 101a, and discharges the gas into a discharge chamber (not shown).
The shoes 106 coupling the piston 105 to the swash plate 100 slide against the swash plate 100 and the walls of the slot 105b. Furthermore, the force of the swash plate 100, which reciprocates the piston 105, and the compressor reaction, which is produced by the compressing action of the piston 105, is applied to a coupling portion of the piston 105, which couples the piston with the swash plate 100. Thus, the coupling portion must be lubricated sufficiently.
Some types of compressors draw refrigerant gas into the suction chamber from an external refrigerant circuit by way of the crank chamber. In such compressors, the refrigerant gas from the external refrigerant circuit passes through the crank chamber. Thus, the lubricating oil suspended in the refrigerant gas, which passes through the crank chamber, lubricates various parts of the crank chamber.
However, the compressor of FIG. 7 is a variable displacement compressor in which the displacement of the compressor is varied in accordance with the inclination of the swash plate 22. In this compressor, the inclination of the swash plate 22 changes in accordance with the difference between the pressure of the crank chamber 103 and the pressure of the cylinder bore, 101a. The pressure difference between the crank chamber 103 and the cylinder bore 101a is altered by adjusting the pressure of the crank chamber 103 with a control valve (not shown) or the like. Accordingly, in such a variable displacement compressor, the refrigerant gas from the external refrigerant circuit is directly drawn into the suction chamber without passing through the crank chamber 103 since the pressure of the crank chamber 103 must be adjusted to control the inclination of the swash plate 100. Therefore, it is difficult to provide the crank chamber 103 of the compressor shown in FIG. 7 with a sufficient amount of lubricating oil. As a result, tho lubrication of the coupling portion of the piston 105 may become insufficient.
The slot 105b provided in the skirt 105a of the piston 105 results in the skirt 105a being thin. In addition, concentrated stress tends to act on the pair of inner corners 105c in the slot 105, and especially, on the inner corner 105c that is farther from the head of the piston 105. For example, when the piston 105 moves from the top dead center position to the bottom dead center position, the swash plate 100 urges the skirt 105a toward the left, as viewed in FIG. 7, by means of the shoes 106. During this action, a force acting to open the slot 105b is applied to the inner corners 105c in a concentrated manner. This may lead to deformation or damage to the skirt 105a and degrade the performance of the compressor.